This invention is concerned with plain bearings, in particular bearings which require to operate with (high) loading levels and low viscosity liquid lubricant.
In particularly heavily loaded bearing conditions wherein a low viscosity lubricant is used, as may be experienced for example in vehicle suspension parts such as dampers, and/or where fluid flows are severe, such as gear pumps, there is a need to provide a bearing surfaced with a lining material that exhibits both fatigue strength and resistance to erosion by such liquid.
Traditionally such bearings have been developed with lining materials of one or more layers of metal alloys chosen for their physical characteristics within the operating environment, but there is developing a trend to avoid using bearing metals as bearing linings, because of both cost and environment factors, and to use polymer based materials as bearing linings.
Plain bearings are known in which a backing strip of steel or comparable metal has formed thereon a lining layer comprising a porous metal matrix of sintered bronze and a low friction polymer-based material that incorporates fillers, hereinafter referred to as a filled polymer, infiltrated into the interstices of the sintered matrix forming a relatively thin overlying skin, or polymer layer.
It is known to infiltrate the low friction polymer-based material as a xe2x80x9cmushxe2x80x9d with an organic lubricant which is pressed into the interstices of the bronze matrix and heated to drive off volatile components and solidify it by a sintering action. It is also known to infiltrate the polymer-based material as a so-called xe2x80x9cdry paperxe2x80x9d.
Such backed, infiltrated sintered bronze bearings are used both in oil lubricated bearings and also in dry bearings, wherein there is minimal lubricating fluid.
Polytetrafluoroethylene, hereafter referred to as xe2x80x9cPTFExe2x80x9d, has good tribological properties and is often used as, or in, a lining material for plain bearings as the main polymer or xe2x80x9cbasexe2x80x9d polymer on which any mixture is based; however, PTFE is soft and weak and has an unacceptably high wear rate when used alone and for practicability requires the addition of so-called wear resistant fillers and/or other materials which reinforce the lining material against rapid wear and low load bearing strength.
Whereas in a dry or marginally lubricated bearing, and to a lesser extent an oil lubricated bearing, wear occurs as rubbing erosion which is directly responsible for removal of the filled PTFE, the rate of removal being determined by the roughness of the surface of the body borne by the bearing and thus the dynamic and break-away friction between the body and bearing, in an oil lubricated bearing, wear results from the lubricating oil bringing about cavitation erosion of the filled PTFE.
However, where fatigue strength and erosion resistance are important, polymer compositions which have been developed for improving either one of these properties have been found to exhibit a worsening of the other property. That is, the polymer compositions, PTFE-based or otherwise, developed for increased strength have been accompanied by reduction in their frictional abilities, i.e., an increased coefficient of friction and lowered erosion resistance, and vice versa.
Patent specification number GB-A-2166142 describes a PTFE-based polymer lined bearing exhibiting enhanced erosion resistant properties by virtue of a filler of an ionic fluoride, particularly a finely divided form such as calcium fluoride.
Patent specification No. GB-A-2279998 (WO95/02772) the contents of which are incorporated by reference, describes a plain bearing of the filled PTFE infiltrated sintered bronze type that is specifically intended as an oil lubricated bearing. The specification summarises the perceived situation in the art regarding reinforcement and filler materials that provide strength and wear resistance, namely stating that the use of xe2x80x9csmoothxe2x80x9d filamentary materials such as glass and aramid fibres per se that do not adhere to PTFE should be considered as unsuitable, before concentrating on the suitability of fibrillated aramid filaments (hereafter also referred to as aramid xe2x80x9cfibresxe2x80x9d) and in addition thereto particulate ionic fluoride fillers to achieve a lining material having improved wear against cavitation erosion. It is found in practice that notwithstanding a wide range of variation in the amount of aramid fibres that provide a useful product, there is in addition to the aforementioned conflict of properties, restrictions brought about by employing such aramid fibres at the upper end of the range as the mush has a tendency to become tough and intractable, making it difficult to spread in other than a thick overlay.
It is an object of the present invention to provide a plain bearing that has a lining layer of reinforced, filled PTFE-based polymer infiltrated into a sintered metal matrix and has a higher fatigue strength and liquid erosion resistance better than hitherto, and a method of manufacturing such a bearing.
According to a first aspect of the present invention a plain bearing comprises a metallic backing, a sintered porous metal layer bonded to the backing and a lining layer infiltrated into the pores of the porous metal layer and overlying the sintered metal layer, said lining layer comprising PTFE containing 10-30% by vol. particulate wear-resistant filler, 2-10% by vol. fibrillated aramid fibres and 2-10% by vol. of a melt processable fluoropolymer.
Preferably the melt processable fluoropolymer is a copolymer of the group tetraflouoethylene-hexafluoropropylene (referred to herein as FEP), tetraflouoethylene-perfluoroalkylvinylether (referred to herein as PFA) and monofluoroalkoxy (referred to herein as MFA). More preferably the melt processable fluoropolymer is a copolymer of said FEP.
Preferably, the melt processable fluoropolymer is present in the lining layer to an amount of 5 to 8% by vol., and more preferably to an amount of 7% by vol. Furthermore, it is preferred that the fibrillated aramid fibre is present in the lining layer in an amount at least 50% by vol. of the melt processable fluoropolymer.
The filler may be any particulate filler known in the art as providing wear resistance in polymer bearing materials. Conveniently, it comprises principally or wholly an ionic fluoride, such as CaF2, MgF2, or SrF2. preferably the inorganic filler is present in the lining layer to an amount in the range 15-20% by vol.
The fibrillated aramid fibres preferably have an average length in the range 0.2 to 1.0 mm and are present in the lining layer to an amount of 3 to 5% by vol.
In respect of the fibrillated aramid fibres and general method of manufacture the invention follows the aforementioned Patent specification No GB-A-2279998, namely the average length of the aramid fibres used in the invention may be 0.2 to 1.0 mm, their diameter prior to fibrillation may be 0.012 mm-0.015 mm, and the degree of fibrillation (as measured as Canadian Freeness) may be 200 (Du Pont method TM 0894-84, Reference TAPPI-T-227M-58).
In respect of the manufacture, and according to a second aspect of the present invention, a method of manufacturing a plain bearing comprises forming a laminar metallic backing strip, bonding thereto by sintering a porous metal layer, forming a mush of fibre-reinforced, filled, PTFE-based bearing material in a volatile lubricant, spreading the mush onto the porous metal layer, pressing the mush to infiltrate a part thereof into the porous metal layer and leave a lining layer overlying the porous metal layer, heating the combination to a first elevated temperature to remove volatile liquid components and a second elevated temperature to effect sintering of the PTFE component and cohesion of the bearing polymer, preparing the mush by mixing together:
(1) an aqueous slurry of particulate, wear resistant filler containing 10-30% by volume solid, (2) an aqueous dispersion of fibrillated aramid fibre containing 2-10% by volume solids, (3) an aqueous solution of melt processable fluoropolymer containing 2-10% by solids, (4) the remainder being an aqueous dispersion of PTFE; adding a lubricant to the mixture, coagulating the mixture to produce a precipitate to effect said spreadable mush form with said lubricant, heating said combination at a said first temperature in excess of 100xc2x0 C. to remove said volatile components and heating said combination a said second temperature between 340xc2x0 C. and 420xc2x0 C. for a heating duration of at least 7 seconds.
Preferably the ingredients (1) to (4) of the preceding paragraph are mixed together prior to addition of the lubricant. In this specification, as in the art in general, the term xe2x80x9clubricantxe2x80x9d is used to refer to a material adsorbable onto the surface of the PTFE to assist in distributing it within a mixture of other ingredients. Commonly and conveniently, toluene may be employed as such lubricant.
Pre-drying the mixture to a mush or paste, that is, removing most of the superfluous liquid at ambient temperature, is achieved by mechanical means such as centrifuging or pressing.
Preferably, to minimise the processing time, the second temperature is in the range 400xc2x0 C. to 420xc2x0 C., and the heating duration is in the range 30 to 50 seconds. Preferably range 44 to 46 seconds.
It has been found that the fatigue strength of the bearing lining as made with any specific formulation within the above mentioned ranges increases as a function of the duration for which it is sintered at elevated temperature.
The strength achieved is believed to be a product of duration and temperature; that is, a fatigue strength can be achieved in short time at higher temperature. This is, of course, contingent upon the perceived degradation of the polymers, particularly the PTFE which begins to become significant at temperatures above 400xc2x0 C. Such degradation is, in practice, not found to present a problem at temperatures above this, even up to 450xc2x0 C., for the durations envisaged to meet typical fatigue strength requirements for these materials.
In a typical manufacturing practice adopted by the applicant, a bearing in accordance with the aforementioned GB-A-2279998 initially at ambient room temperature has been passed in strip form through a continuous oven, of length 1.5 m and heated to approximately 400xc2x0 C., at a travelling speed of 3 m/min such that, assuming that lining materials reach the oven temperature at approximately half distance, the lining is subjected to the 400xc2x0 C. temperature for about 15 seconds.
It is found practicable, and in industrial terms cost effective, to sinter the polymer based bearing lining in accordance with the present invention at said second elevated temperature for the order of three times the aforementioned duration, about 45 seconds, to achieve significant increase in the fatigue strength of the lining, although this is subject to considerable variation, both in respect of duration and temperature, in accordance with time constraints imposed on the manufacturing process and properties required of the finished bearing. Such sintering may be performed in such a continuous oven by reducing the travelling speed and/or increasing the oven length, or may be performed as a batch process when longer times are readily attained.
The mush of lining material may optionally have added to it a pigment that gives a specific appearance to the finished bearing. The pigment may be any material commonly made available for such purpose that has no chemical deleterious effect on the lining, nor physically deleterious effects on the bearing properties, and which is able to withstand the relatively high PTFE sintering temperature for whatever duration and temperature are chosen to achieve the desired fatigue strength.
Is it known from GB-A-2196876 to enhance cavitation resistance in a PTFE-based lining polymer by incorporating a percentage of FEP (and/or PFA) and lead-tin alloy which effect between them a reported increase in wear strength and erosion resistance. An inter-polymer is formed as particles which serve as individual carriers of small particles of the lead-tin alloy to effect distribution of this heavier component throughout the lighter polymer components, and it is the alloy-carrying inter polymer particles which are pressed into a porous metal substrate and heated to produce the bearing.
The presence of the FEP (and/or PFA) is shown to increase the fatigue strength of the lining material but its presumed merits are compromised by a higher coefficient of friction than PTFE and poor wettability by low viscosity lubricants. The poor wettability and frictional properties are mitigated by the use of the lead-tin alloy.
In a metal-free lining layer there is not only the problem of the relatively poorer frictional properties of FEP (or other heat processable fluoropolymer) in the absence of the soft bearing metals but also the difficulties which result from using it in a wet lining mush rather than as the specific particulate form described in the above publication. In particular it is found to make the mush particularly slimy and difficult to handle on an industrial scale and with current manufacturing plant. The exact reason for this is not clearly understood, but it is believed to be due to the use of surfactants with the commercially available fluoropolymers.
However, the lining as defined in the present invention has been found to have acceptable frictional properties with superior fatigue strength and erosion resistance without the use of metallic components, and furthermore, by having a suitable percentage of fibrillated aramid fibres in relation to the heat processable fluoropolymer, about one half of the volume of the heat processable fluoropolymer, the mush resulting from their mixture in aqueous form and with an organic lubricant, is not excessively slimy and the resultant mush form is readily handled and worked using conventional equipment and procedures.